In adult mammals, the insulin gene is expressed only in the pancreatic β-cells in the islets of Langerhans. The proximal 5′-flanking region of the insulin promoter has been shown to be sufficient for directing β-cell-specific expression of the insulin gene (Dandoy-Dron et al. (1991) Nucleic Acids Res. 19, 4925-4936; Edlund et al. (1985) Science 30, 912-916; Hanahan (1985) Nature 315, 115-122; Crowe et al. (1989) Mol. Cell. Biol. 9, 1784-1789; Stellrecht et al. (1997) J. Biol. Chem. 272, 3567-3572; Stein (1993) Trends Endocrinol. Metab. 4, 96-100; Sander et al. (1997) J. Mol. Med. 75, 327-340). Further, mutational analysis of the promoter proximal region has identified several cis-acting enhancer elements that are important for insulin expression.
Three conserved insulin enhancer elements, A3 (−201 to −196 bp), E1 (−100 to −91 bp), and RIPE3b/C1-A2 (−126 to −101 bp) play an important role in regulating cell-specific expression of the insulin gene. Transcription factors that bind and activate expression from two of the three conserved insulin enhancer elements (A3 and E1) have been cloned. PDX-1, a member of the homeodomain family of transcription factor expressed in cells of the pancreas and duodenum, binds the A3 element. Heterodimers of ubiquitously distributed basic helix-loop-helix family members (E2A and HEB) and the cell type enriched basic helix-loop-helix member (BETA2) bind the E1 element. A RIPE3b binding activity has been detected in nuclear extracts from both insulin-producing and non-insulin-producing cell lines (Shieh et al. (1991) J. Biol. Chem. 266, 16708-16714; Robinson et al. (1994) J. Biol. Chem. 269, 2452-2460; Zhao et al. (2000) J. Biol. Chem. 275, 10532-10537; Shieh et al. (1995) J. Biol. Chem. 270, 21503-21508; Sharma et al. (1995) Mol. Endocrinol. 9, 1468-1476; Sharma et al. (1994) Mol. Cell. Biol. 14, 871-879. Two specific RIPE3b-binding complexes have been identified: 1) a cell-specific complex, RIPE3b1, which is detected only in pancreatic β-cell lines, and 2) the RIPE3b2 complex that is detected in nuclear extracts from all cell lines examined to date.